The present invention relates to a demodulating device and, particularly, to a demodulating device for demodulating a signal whose carrier frequency variation is considerable.
In order to demodulate an input signal such as an intermediate frequency (IF) signal or any other modulated signal to produce a baseband signal, a demodulating device composed of a non-coherent demodulator, a coherent demodulator and a phase-locked loop (PLL) type carrier reproduction circuit has been known. In such conventional demodulating device, the non-coherent demodulator multiplies a modulated IF signal with a local oscillation frequency signal whose frequency is close to that of the modulated IF signal to produce a complex baseband signal containing frequency error. Then, the complex baseband signal is multiplied with a reproduced carrier signal in the coherent demodulator to produce a baseband signal. The carrier is reproduced by using PLL type carrier reproduction circuit. The carrier reproduction circuit serves to synchronize an oscillation frequency of a voltage controlled oscillator with the frequency error components of the baseband signal. An example of such PLL type carrier reproduction circuit is described in pages 222-225 of a book entitled "Phase-lock Techniques" by Floyd M. Gardner, Ph.D and published John Wiley & sons, Inc., reprinted 1979.
When such demodulating device is used to demodulate a modulated signal such as used in mobile communication system in which carrier frequency may vary considerably, the frequency variation may go out of a pull-in range of the PLL carrier reproduction circuit, so that a reproduced carrier may frequently slip out, making it impossible to receive the modulated signal. On the other hand, when, in order to make the pull-in range wider to solve this problem, a loop noise bandwidth of a loop filter provided inherently within the PLL type carrier reproduction circuit is expanded, noise becomes considerable, causing insensitivity of the demodulating device to noise to be degraded.